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Physics Revision-II

If you have specific Questions/Numerical problems in which you need help please post them here: Physics Forum

Electricity and Magnetism

Questions

What resistance must be placed in parallel with 20Ω to make the combined resistance 15Ω?

A heater is labeled 1600 W/120 V. How much current does the heater draw from a 120-V source?

We desire to measure the current through and the voltage across a resistor connected in a circuit. How should an ammeter and a voltmeter be connected to the resistor?

What is the current through an 8.0-Ω toaster when it is operating on 120 V?

Three resistors, of 8.0 Ω, 12 Ω, and 24 Ω, are in parallel, and a current of 20 A is drawn by the combination. Determine (a) the potential difference across the combination and (b) the current through each resistance.

What happens to the direction of the magnetic field about an electric current when the direction of the current is reversed?

Why there is more resistance when you attempt to move a magnet into a coil of 10 loops than a coil of 5 loops.?

What must change in order for electromagnetic induction to occur?

In addition to induced voltage, what does the current produced by electromagnetic induction depend?

What are the three ways that voltage can be induced in a wire?

How does the frequency of induced voltage compare to how frequently a magnet is plunged in and out of a coil of wire?

What is the basic similarity between a generator and an electric motor?

What is the basic difference between a generator and an electric motor?

Why does a generator produce alternating current?

Is electromagnetic induction a key feature of a transformer?

Why does a transformer require alternating current?

What is the principal advantage of ac over dc?

What are the characteristics of a step down transformer?

What will be the magnitude of currents in a secondary coil when steady current flows in a nearby primary coil? Explain.

What is Magnetic flux?

A bar magnet is inserted in a solenoid. Will the speed of movement of the bar magnet to and fro inside the solenoid effect the current induced?

Heat and Calorimetry Questions

Why is water preferred in hot water bag than other liquids

Which material will have a quicker rise in temperature… a material A with high specific heat and the other material B with a low specific heat?

Can pressure be measured (approximately) by a mercury thermometer?

Is the specific heat of ice same as that of water?

What is the heat required when ice at -2 degC converted to water at 14 degC.

Why the melting point of water is lowered with rise of pressure? Give an example where melting point is increased with rise in pressure

Which is more injurious, water at 100 degC or steam at 100 degC?

Can the freezing point and boiling point of water be same?

Are melting point and freezing point same?

How do skates skid on ice?

What will be the effect of addition of salt on the freezing point and boiling point of water?

Why water stored in an earthen pitcher is cool?

Is it possible to boil water by passing steam at 100 degC over water?

A copper ball of mass 100 gm and temperature 100 degC is dropped in 50 gm of water at 30 degC. Find the final temperature of the mixture, assuming sp. Heat of copper is .1 and heat transmitted to container and surrounding is zero.

The ratio of densities of two metals A and B is 3:4 and the ratio of the sp. Heat is 5:4. Find the ratio of heat capacities of unit volume of two metals.

Electricity and Magnetism Answers

60.0 ohm

13.3 A

The ammeter is connected in series and the voltmeter is connected in parallel with the resistor

15 A

(a) 80 V (b) 10 A, 6.7 A, 3.3 A

The direction of the magnetic field gets reversed too.

Each loop creates an electromagnetic force. More the number of loops, the stronger the electromagnetic force.

The magnetic field must change for the electromagnetic induction.

The resistance of the coil and the circuit determines the current produced.

Three ways are by moving the loop near a magnet, moving a magnet near the loop, or by changing a current in a nearby loop.

The same, for the frequency of the alternating voltage induced equals the frequency of the changing magnetic field.

The construction of each is identical. They look the same.

The roles of input and output are reversed. In a motor, electric energy is the input and mechanical energy the output; in a generator, mechanical energy is the input and electric energy the output.

Because the changing magnetic field alternates, making the induced voltage alternate, this makes the current alternate.

Yes

Alternating current provides the necessary ingredient for induction–change.

The ease with which voltages can be stepped up or down with a transformer.

Primary coil has more loops than secondary, primary coil has more voltage than the secondary coil.

The current in the primary will be zero. This is because the steady current does not induce an emf. For induction of emf the current needs to vary with time.

Magnetic flux is the intensity or the number of magnetic lines of force passing through a plane.

Yes, the faster the rate of movement, the more emf will get induced. This is because the rate of change of magnetic flux induces emf. More rapid change of flux will generate stronger emf.

Heat and Calorimetry Answers

1. The specific heat of water is highest among all the substances. Thus a certain mass of water heated to a certain temperature contains more heat than the same mass of any other liquid heated to the same temperature. So hot water will take longer time to cool than any other liquid heated to the same temperature .So hot compression can be continued for a sufficiently long time.

2. Specific heat of A is more than that of B, so A will require more heat for the same rise of temperature. Therefore for the same supply of heat the temperature rise of B will be higher than that of material A.

3. The boiling point of water decreases by 1 degC for per 27mm decrease of pressure (27mm decrease means 27mm of mercury). So, if water boils above 100 degC, atmosphere is above its normal value i.e. above 76 cm of mercury, if water boils at a temperature below 100 degC , the atmospheric pressure is below 76 cm of mercury.

For instance , if the thermometer shows that the boiling point of water at a place is 102 degC, then at that time the atmospheric pressure is (76 cm + 27mm*2)or 81.4 cm of mercury. Similarly for a boiling point of 96degC the atmospheric pressure is (76 cm – 27 mm *4) or 65.2 cm of mercury.

4. No. Specific heat of ice is lower than that of water though they are chemically same. This is because they are of different phase. The specific heat of water is 1 cal/gm/degC. While that of ice is about 0.5 cal/gm/degC

5. When ice at -2 degC is being heated then the heat will be taken to increase the temperature of water from -2 degC to 0 degC. When the ice comes to 0 degC then the heat supplied is absorbed to change the state of ice i.e. to convert ice to water by taking the latent heat of fusion. When the whole mass of ice is converted to water then the heat supplied will increase the temperature of water by taking the specific heat .

6. Ice on melting decreases in volume.12 cc of ice on melting gives 11 c of water. That means the molecular distance decreases on melting. Increase of pressure creates the same effect thus lower latent heat energy is required for bringing the molecules closer. So melting point is lowered with increase in pressure.

For substances which increase in volume on melting, like wax etc., the melting point increase with increase in pressure. In these cases the molecules distance each other on melting. Pressure creates reverse effect i.e. bring them closer. Thus extra latent heat is to be provided resulting in the rise of melting point.

7. Steam at 100 degC is more injurious than water at 100 degC. This because the same mass of steam will contain more heat than same mass of water. Steam at 100 degC contains the heat contained in water at 100 degC plus the latent heat required(540 cal/gm) for the conversion of water to steam. So steam will cause worse injuries.

8. Yes. The boiling point and the freezing point of water can be same. It is called triple point of water. At very low pressure (611 pa, at about .006 times atmospheric pressure) both boiling point freezing point is equal to .01 degC.

9. No. the freezing point and the melting point are not same. They are assumed and are approximately same for most of the materials. In case of pure mercury they are equal (-38.83 degC).for water at normal atmospheric pressure both are equal to 0 degC.

For agar they are different. It melts at 85 degC and melts at about 30 to 41 degC.

10. Water has its melting point lowered due to increase of pressure. Water Liquefy under increased pressure and resolidify on removal of the excess pressure. When the skater skates the weight of the skater and the skates liquefy the ice under the skates. As soon as the skate has moved, the pressure over the ice is not there and it resolidifies.

11. On addition of salt in water the freezing point is dropped.

The melting point is increased with the increase with addition of salt. Normal water boils in 100 degC but sea water boils at about 104 degC.

12. There are a large number of pores in an earthen pitcher. When water is poured in it, some of the water trickles out slowly through these pores. The water droplets on coming in contact with air start evaporating. The latent heat of evaporation is partly taken from the pitcher and mainly from the water inside the pitcher. Due to this loss of heat the water cools.

13. No. by passing steam at 100 degC into the water the temperature of water can be raised up to 100 degC at most, but each gram of water will require more 540 cals of heat to convert from water to steam. Now as both steam and water are at same temperature , so heat will not flow between them as principle of calorimetry says heat flows from a body at higher temperature to a body at lower temperature. Thus water will not boil.

14. 41.66 degC

15. 15:16

If you have specific Questions/Numerical problems in which you need help please post them here: Physics Forum

Sir can u help me to solve this kind of qns……..(The subscript nd superscripts are given as simple numbers):-
Uranium nucleus 238U92 decays to lead nucleus 206Pb82. How many α and β particles are emitted?